Tunable optical filtering component

ABSTRACT

The invention relates to wavelength-selective and tunable optical filters for transmitting the light in a narrow optical spectral band, centered around an adjustable wavelength, and for blocking the transmission of wavelengths lying outside of this band. In a micromachined monolithic structure containing the optical filter proper, the component comprises a low-absorption light detection element used for slaving the tuning control of the filter to a wavelength received by the filter, this element transmitting the majority of the radiation at this wavelength. The filter is a Fabry-Pérot interferometric filter, the cavity (C) of which is tuned to a value that maximizes the power detected by the light detection element. The filter is preferably based on layers of indium phosphide and air gaps. The detection element preferably comprises a layer of gallium-indium arsenide 74 suitable for detection of the intended wavelength band.

The invention relates to wavelength-selective and tunable opticalfilters for transmitting light in a narrow optical spectral band,centered around an adjustable wavelength, and for blocking thetransmission of wavelengths lying outside of this band. The centralwavelength of the narrow spectral band is adjusted by electrical means.

The word “light” is intended-in the wide sense and includes, inparticular, spectral bands in the infrared as will be seen below, amajor application of the invention being to filter light in the variousfiber-optic telecommunication bands lying between 1.3 and 1.61micrometers.

The advantage of these 1.3 to 1.61 micrometer bands results from thefact that current optical fibers, made of glass, have low attenuationand the optical signals can therefore be transmitted over largedistances. In what follows, the invention will be explained withreference to this spectral band, although it should be understood thatthe invention may be applied to other bands if the need arises, by usingmaterials suitable for these different bands.

In a fiber-optic telecommunication network, a cable comprising aplurality of optical fibers can be used to form a plurality of differenttransmission channels; time division multiplexing of the information mayalso be carried out in order to achieve the same objective; with a viewto further increasing the information delivery capacity of the network,however, the current trend is for a plurality of light wavelengths,modulated independently of one another and each defining an informationchannel, to be transmitted simultaneously on the same optical fiber. ITU(International Telecommunications Union) Standard 692 proposes thedefinition of adjacent channels with an optical spectral bandwidth of100 GHz, centered on N adjacent standardized optical frequencies whosevalues are 200 terahertz, 199.9 terahertz, 199.8 terahertz, etc.,corresponding to N wavelengths of from 1.52 micrometers to 1.61micrometers. Modulation of the light at from 10 to 20 gigabits persecond can be carried out on a channel having this bandwidth, withouttoo much risk of interference between the immediately adjacent spectralbands (by using modulation pulses of Gaussian shape in order to minimizethe passband occupied by this modulation). This technique of frequencydivision multiplexing is referred to as DWDM, standing for “DenseWavelength Division Multiplexing”.

In a telecommunication network, the problem is therefore that of beingable to collect the light corresponding to a determined channel withoutperturbing the light of the neighboring channels. At a transmission nodeof the network, which is assigned to the transmission and reception ofinformation on channel i, for example, it is necessary to be able tocollect the light at the central frequency F_(i) (wavelength λ_(i))without impeding transmission of the light modulating the centralfrequencies F₁ to F_(N), even though these optical frequencies are veryclose together.

To that end, there is a need to produce optical filtering componentswhich are highly selective for light wavelengths and are capable oftransmitting the central optical frequency F_(i) and the frequencieslying in a narrow band of less than 50 GHz on either side of thisfrequency, and of blocking the other bands. Only the light of channel iis collected at the output of such a filter, and this can be demodulatedin order to collect the useful information.

It has already been proposed to produce filtering components thatoperate on the principle of Fabry-Pérot interferometers, which areproduced by depositing semiconductor layers separated from one anotherby air gaps with thicknesses calibrated according to the wavelengthλ_(i) to be selected. In practice, an interferometer comprises twomirrors made of stacked dielectric layers (Bragg mirrors) with a highcoefficient of reflection, which are separated by a transparent zonewith an optical thickness of k·λ_(i) (real thickness k·λ_(i) if the zoneis an air gap), where k is an integer defining the order of theinterferometric filter. Indium phosphide (InP) is highly suitable forthese embodiments, in particular because of its transparency for thewavelengths in question, its very high refractive index and thepossibility of depositing epitaxial layers with a well-controlledthickness.

If the thicknesses of the layers and the intervals between layers arevery well controlled, and if the materials have a high refractive index,such a filter turns out to be highly selective.

Such an embodiment is described in the article by A. Spisser et alii,“Highly Selective 1.55 micrometer InP/airgap micromachined Fabry-Pérotfilter for optical communications” in Electronics Letters, No 34(5),pages 453-454, 1998. Other embodiments, made of micromachined siliconand of alloys based on gallium arsenide, have been proposed.

These filters may be tunable by varying the thickness of the Fabry-Pérotresonant cavity, that is to say the transparent zone separating the twomirrors. The cavity is delimited by two opposing semiconductor layers,the spacing of which is defined very precisely during fabrication; bymaking an electrical contact on each of them (the layers being assumedto be sufficiently conductive or coated with a conductive material), aDC voltage can be applied that creates electrostatic forces between theopposing layers, tending to modify the spacing in a controlled way.

It has been shown that it is possible to produce interferometric filterswhose layers are suspended from micromachined suspension arms with athickness small enough so that a voltage of a few volts is sufficient tomodify the tuning of the filter over about one hundred nanometers, forexample throughout the wavelength range of the standardized band of from1.52 to 1.61 micrometers. The nominal thickness of the cavity is 0.785micrometers, for example, and applying a voltage of the few volts makesit possible to vary this thickness by 100 or 200 nanometers up or down,which is sufficient to modify the tuning of the filter throughout thespectral band of the ITU standard. In practice, the tuning may bemodified by 1 GHz per millivolt, which is very satisfactory since it isthen possible to change channel with a control voltage modification offrom 50 to 100 millivolts.

It is, then, sufficient to establish a correspondence table between achannel number (and therefore a central wavelength) and a controlvoltage to be applied for selecting any one of the channels, and to sendthe output of the filter to a demodulator (photodetector) which convertsthe information carried by this channel into an electrical signal.

It will, however, be understood that the tuning conditions would be easyto control if the central frequencies were far apart, for exampleseparated by 1 terahertz, but that they are much more difficult tocontrol when the spacing is only 100 gigahertz. This is because even attransmission, the frequency of a laser which emits the carrier ofchannel i may experience fluctuations and drifts due to temperature orto ageing (of the order of a few tens of gigahertz).

It is therefore desirable to slave the control voltage of the filteronce it has been locked onto the correct central frequency, in order tomaintain this voltage subsequently.

The slaving systems which have been provided are bulky and expensive toproduce, in particular because of the opto-mechanical componentsnecessary for the operation (couplers, optical fiber connectors,mirrors, etc.).

The present invention provides a tunable optical filtering componentwhich comprises, in a micromachined monolithic structure containing theoptical filter proper, a low-absorption light detection element used forslaving the tuning control of the filter, this element transmitting themajority of the filtered light and therefore substantially notperturbing the optical transmission of the filter.

This detection element collects a very small fraction of the lighttransmitted in the intended band, and converts this fraction into anelectrical signal which can therefore be used, after averaging over acertain time, as an electrical control signal for slaving that tends tomaintain the tuning of the filter at a value which maximizes theaveraged detected signal. The majority of the light at the selectedwavelength passes through the filter and the detection element, and itcan be used elsewhere, in particular for purposes of modulating theuseful signal. Because of its lower absorption, the detection elementplaced in the monolithic structure does not perturb the transmission ofinformation, and it is only used for purposes of slaving the tuning ofthe filter itself. The slaving is carried out on the received light andtherefore takes into account the wavelength fluctuations attransmission.

The component according to the invention preferably includes atransparent semiconductor substrate (the concept of transparency relatesof course to the wavelengths of the band in question), on the front faceof which a stack of likewise transparent layers is formed constituting atunable interferometric filter that selectively transmits the light in anarrow spectral band, centered on a wavelength which can be adjusted byan electrical voltage, the light detection element being formed on thefront face of the substrate, between the substrate and the filter. It isalso possible to envisage a configuration in which the illumination iscarried out via the rear face of the substrate, the detection elementthen being placed above the filter.

The substrate is preferably made of indium phosphide, and theinterferometric filter includes a plurality of indium phosphide layersseparated by intervals of controlled width, at least one of whichintervals has a width that can be varied under the control of anelectrical voltage in order to tune the filter. The intervals arepreferably produced in the form of air gaps, although some of them maybe filled with a transparent material having a refractive indexdifferent to that of the indium phosphide layers.

The photoelectric detector is preferably produced by means of aquantum-well photodiode. The latter consists of at least one P-typedoped semiconductor layer, a semiconductor layer that is notintentionally doped, and an N-type doped semiconductor layer, thesethree layers being semiconductor epitaxial layers (lattice-matched ornearly lattice-matched with the indium phosphide), and a very thinepitaxial layer of a different semiconductor alloy, which is insertedinto the layer that is not intentionally doped. This very thin layer ismade of a material having an energy “gap”, that is to say an energyinterval between the conduction band and the valence band, of about0.775 electron volts, corresponding to strong absorption in the opticalwavelength band of from 1.5 to 1.6 micrometers. The thickness of thevery thin epitaxial layer is small enough to avoid the generation ofdislocations by plastic relaxation of this layer, in spite of thecrystal lattice differences liable to exist between this layer and thesemiconductor layers (principally indium phosphide) which enclose it.This thickness is preferably less than 10 to 20 nanometers, in order tolimit the absorption of light to a value of the order of 1 percent.

By using a quantum-well detector whose absorption is low because of thevery small thickness of the absorbent layer, the transmission of thelight flux in the monolithic component is not perturbed. The absorptionis preferably less than 1 to 2% in total.

The semiconductor alloy layer constituting the quantum well in the InPlayer is preferably made of In_(x)Ga_(1-x)As, where x is selected sothat the gap of the material, corrected for quantization and straineffects, is about 0.775 electron volts in order to absorb wavelengths upto about 1.63 micrometers. The number x is preferably 0.532. It may varyslightly around this value, preferably in the range of from 0.53 to0.63, in which case the layer has a mismatched crystal lattice inrelation to the indium phosphide and its thickness should be limited tofrom 5 to 20 nanometers. The semiconductor alloy could also be InGaAsP.

Other characteristics and advantages of the invention will becomeapparent on reading the following detailed description which is providedwith reference to the appended drawings, in which:

FIGS. 1 to 3 represent the schematic structure of a monolithicinterferometric filter according to the invention, respectively in crosssection, in plan view and in perspective,

FIG. 4 represents a circuit diagram for optical frequency slavingassociated with the filter.

The invention will be described in relation to a filter formed on anindium phosphide substrate, for an application to fiber-optictelecommunications in a broad spectral band of from 1.52 to 1.61nanometers.

The filter is designed, on the one hand, in order to transmit a narrowspectral band around a wavelength X_(i) and, on the other hand, in orderto reflect the wavelengths outside this band. It is tunable in order togive the wavelength X_(i) any value throughout the broad spectral band.

To that end, it consists of a Fabry-Pérot resonant cavity, that is tosay a cavity (C) of air or a medium with an index n₀ between twoparallel dielectric mirrors M1 and M2. The distance D between themirrors is an integer multiple (k) of half the wavelength to beselected: D=kλ_(i)/2n₀. The mirrors each consist of an alternatingsequence of a plurality of dielectric layers with different refractiveindices and well determined thicknesses. In principle, k is selected tobe equal to 1 (cavity with single-mode resonance) because, if k isdifferent than 1 (cavity with multimode resonance), the cavity will betransparent not only for λ_(i) but also for other wavelengths separatedfrom one another by λ_(i)/k.

The more reflective the mirrors are, the higher is the selectivity ofthe filter for the selected wavelength. The mirrors are commensuratelymore reflective as the difference in the refractive indices of thealternating dielectric layers is high. In the preferred example which isdescribed, the alternating sequence consists of layers of indiumphosphide (refractive index n₁ greater than 3, and of air, which is veryfavorable although other pairings of transparent dielectric layers couldbe envisaged. The reflection is commensurately more independent of thewavelength as there are few layers in the alternating sequence. A highdifference in indices between the layers of the alternating sequence,however, makes it possible to obtain very high reflection even with avery small number of layers in each mirror.

In particular, it is possible to produce the mirrors M1 and M2 with onlytwo layers of indium phosphide each, separated by a layer of air.

Lastly, a high reflection of the mirrors is achieved by an expedientselection of the relative thicknesses of the layers of the alternatingsequence, taking into account their respective refractive indices: theoptical thickness of each layer, that is to say the real thicknessdivided by the optical index, is an odd multiple of one quarter of thewavelength for each of the layers of air or indium phosphide. Theaverage wavelength λ_(m) (about 1.55 micrometers) of the broad spectralbrand in which the filter operates will preferably be used for thesethickness selections.

In the example of FIG. 1, the lower mirror M1 consists of two layers ofindium phosphide 10 and 12 (refractive index n₁) with an opticalthickness 5λ_(m)/4, and therefore a real thickness 5λ_(m)/4n₁, which areseparated by a layer of air 14 (index n₀) with an optical thickness andreal thickness λ_(m)/4.

Like the mirror M1, the upper mirror M2 consists of two layers of indiumphosphide (30 and 32) (refractive index n₁) with an optical thickness5λ_(m)/4, and therefore a real thickness 5λ_(m)/4n₁, which are separatedby a layer of air 34 (index n₀) with an optical thickness and realthickness λ_(m)/4.

The parallel-faced cavity C is an air cavity between the layers 12 and30, the thickness of which is λ_(i)/2 in order to obtain selectivefiltering (single-mode) at the wavelength λ_(i). It will be seen thatthis thickness is adjustable around a nominal design value D. The designvalue D may be λ_(m)/2, in which case the thickness needs to be variedpositively and negatively in order to obtain tuning throughout the broadspectral band. It is also conceivable that the nominal thickness D maycorrespond instead to tuning at one of the ends of the broad spectralband, and for this thickness to be varied in only one direction in orderto cover the entire band.

This filter therefore consists of an alternating sequence of paralleltransparent indium phosphide layers, separated by layers of air, and itis produced by micromachining a monolithic indium phosphide substrate 40by suitable operations of epitaxial layer deposition and etching on thesubstrate. The fabrication, which uses in particular the deposition ofsemiconductor materials other than indium phosphide, in particulargallium-indium arsenide, will be discussed further on. These materialsremain present on the substrate throughout the resonant-cavity filterand constitute spacers, which support the indium phosphide layers andwhich define the thickness of air gaps. These materials are selected onthe one hand for their semiconductor properties and, on the other hand,for their ability to be etched selectively with respect to the indiumphosphide.

The lower layer 10 of the mirror M1 is preferably separated from thelast epitaxial layer 42 deposited on the underlying substrate 40, by alayer of air with an optical thickness close to λ_(m)/2 or a multiple ofλ_(m)/2 in order to facilitate transmission of the wavelength λ_(i)toward the substrate. The epitaxial layer 42 is used as a support basefor the filter deposited above.

The light arrives from the top (on the same side as the mirror M2), forexample through an optical fiber carrying a plurality of opticaltelecommunication channels, including one channel i at the wavelengthλ_(i); this wavelength is selected by the interferometric filter(mirrors M1, M2, cavity C), and only the narrow spectral band aroundλ_(i) passes through the filter and reaches the indium phosphidesubstrate 40. The substrate is transparent and the light is collected onthe other side of the substrate, where the information present inchannel i can be detected and processed. The rest of the light isreflected by the Fabry-Pérot filter.

The filter is made tunable in the following way: the various indiumphosphide dielectric layers are constructed in the form of centralsurfaces 45 (for example disk-shaped) suspended by suspension arms 50,as can be seen in FIGS. 2 and 3. The stiffness of these arms issufficient to keep the layers accurately parallel to one another, but itis low enough so that, taking into account the distance λ_(m)/2 betweenthe two indium phosphide layers 12 and 30 which enclose the cavity C, itis possible to move these layers toward one another by electrostaticforces produced on these layers, against the stiffness of the arms 50. Afew volts applied between the layers may be sufficient to modify thedistance between the layers by 5% or more, depending on the stiffness ofthe arms.

The layers are semiconductor layers, and they are doped in order fortheir natural conductivity to be high enough so that it is unnecessaryto provide electrodes on the layers. Electrical contacts are establishedon the layers in order to apply the appropriate potential differencebetween them, through the suspension arms. The figures represent anelectrical contact 60 for connection to the layer 30 (through the layer32 and a spacer layer of gallium-indium arsenide, as mentioned above)and an electrical contact 62 for connection to the layer 12 (likewisethrough a plurality of other layers).

The determined potential difference corresponds to a determined spacingD+δD between the layers 12 and 30, and therefore to a determined cavitythickness and hence to transmission for a determined wavelength. It istherefore possible to establish a functional table for selection of agiven optical channel by applying the predetermined potential differencebetween the contacts 60 and 62, which corresponds to tuning on thischannel.

According to the invention, a photoelectric detector which forms part ofthe monolithic component, and which is produced by depositing andetching layers on the substrate, is interposed between the substrate 40and the alternating sequence of dielectric layers which forms theinterferometric filter. This detector is preferably one with very lowabsorption for the wavelengths in question (1.52 to 1.61 micrometers),but not with zero absorption. The absorption is preferably about 1 to2%.

The detector delivers an electrical signal representing a small fractionof the light energy transmitted by the filter. If this electrical signalis filtered by sending it through a lowpass filter, for example a filterwith a time constant of from 1 microsecond to a millisecond, that is tosay one which is long compared with the modulation period (less than onenanosecond) of the information present in the channel, it is possible todetermine an average power passing through the filter. This averagepower is larger if the filter is well tuned than if the filter is poorlytuned. The signal output by the photoelectric detector therebyintegrated in the monolithic component, is used as an input signal forthe circuit for slaving the tuning of the filter. The slaving isdesigned so that the filter is tuned to a wavelength such that theaverage power collected by the detector is maximal. Since there is ofcourse a maximum for each active transmission channel, the slaving isinitialized by tuning the filter approximately to a channel (for exampleto within 50 gigahertz) and by preventing tuning voltage excursionsextending beyond a limit corresponding to a tuning frequency shift ofabout 50 gigahertz. After this initialization on a given channel, theslaving fulfills its function by keeping the filter exactly tuned to theincident optical frequency, in spite of the variations in this frequencyat transmission.

The detector is therefore integrated in the monolithic component, but itdoes not prevent almost all of the light power available in channel ifrom being transmitted onto the other side of the substrate 40, afterhaving passed through the filter. This light power can be processed asrequired.

In order to produce such a low-absorption integrated photoelectricdetector, although indium phosphide is transparent for the wavelengthsin question since it has a gap of about 1.3 electron volts, it ispreferable to produce a quantum-well detector consisting essentially ofan indium phosphide PIN diode, into which a very thin additionalepitaxial layer (quantum layer) of a ternary compound (such as InGaAs)or a quaternary compound (such as InGaAsP) has been inserted. Thecompound is such that it has a gap corresponding to high absorption ofthe useful wavelengths of from 1.52 to 1.61 micrometers. It is also suchthat its crystal lattice is similar to that of indium phosphide. Itsthickness is small enough to allow epitaxial deposition of this layer onthe indium phosphide without dislocation, even if there is a crystallattice difference. The thickness is preferably not more than from 5 to20 nanometers, depending on the composition of the layer.

The very thin quantum layer lies in the middle of the intrinsic layer Iof the PIN diode. One of the layers P or N of the PIN diode may beconnected directly or indirectly to the contact 62, in order to permitbiasing of the PIN diode and collection of a detection signal; if thecontact 62 cannot be used for this purpose (it is already being used toapply the tuning voltage of the filter), then it is necessary to providean additional contact dedicated to making a first contact for the PINdiode. A second contact 64 may be provided on the other side of the PINdiode.

The details of the epitaxial layers deposited in the preferred examplebeing described are as follows, the substrate 40 preferably being madeof indium phosphide that is not intentionally doped (n.i.d.) or ofsemi-insulator, so that it is indeed transparent (doping increases theabsorption of the photons).

p⁺ doped InP epitaxial layer 70, on which the contact 64 can be madedirectly, forming the P layer of the PIN diode;

InP epitaxial layer 72 forming a first part of the intrinsic layer I ofthe PIN diode;

quantum layer 74: a very thin epitaxial layer of gallium-indiumarsenide, with the formula In_(x)Ga_(1-x)As, where x is selected inorder to obtain an energy gap of about 0.7 volts between the valenceband and the conduction band, which allows good absorption of thephotons in the broad spectral band of from 1.52 to 1.61 micrometers; xmay be equal to the value 0.532, which is conventionally selected whenwishing to match the crystal lattice of gallium-indium arsenide exactlyto the lattice of indium phosphide. The thickness of the layer may beabout 15 nanometers in this case, so as to obtain about 1% absorption bythe detector. Different values of x may be selected, however, inparticular in the range x=0.53 to 0.65. In this case, the crystallattice is slightly mismatched in relation to that of the indiumphosphide, and the thickness of the layer will accordingly be limited inorder to avoid dislocations; this thickness limitation is moreover notproblematic in terms of the absorption, which increases when xincreases. For x=0.55, a thickness of 10 nanometers ought to be suitablefor 1% absorption. For x=0.625, the thickness of the layer should belimited to 5 nanometers, again allowing about 1% absorption. Even ifthere is lattice mismatching, the small thickness of the quantumepitaxial layer allows the deposition of this layer to follow thelattice of the indium phosphide epitaxially (albeit with strains); withthis thickness, the strains do not engender any dislocations;furthermore, the small thickness of gallium-indium arsenide leads to lowabsorption of the light, making it possible both to create a usefuldetection signal and to keep the majority of the radiation for anotheruse;

indium phosphide epitaxial layer 76, not intentionally doped,constituting the second part of the intrinsic layer I of the PIN diode;the epitaxial growth maintains the original crystal lattice of thesubstrate 40, since the gallium-indium arsenide layer has also kept toit;

aforementioned epitaxial layer 42 of n⁺ doped indium phosphide formingthe last layer before formation of the Fabry-Pérot cavity; the contact62 is made on this layer so that the contacts 62 and 64 respectivelygive access to the P and N layers of the PIN diode, allowing it tooperate as a detection photodiode.

FIG. 4 represents the way in which the electrical contacts of thecomponent may be inserted into a circuit for slaving the tuning of thefilter: the PIN diode, reverse-biased by a biasing circuit 80, deliversa signal in the form of a voltage or current to the detection circuit82. The detection circuit delivers a voltage representing the quantityof light instantaneously passing through the quantum layer 74. Thisvoltage, optionally amplified by an amplifier 84, is applied to alowpass filter 86 whose time constant is much longer than the period ofthe pulses modulating the light of the optical channel at the wavelengthλ_(i). The time constant may, for example, be 1 microsecond or more.

The output voltage of the lowpass filter is applied to a slaving circuitproper 88, which delivers a signal tending to maintain the outputvoltage of the lowpass filter at a maximum value. Such a circuitestablishes a control voltage to be applied to the optical filter, as afunction of the sign of the derivative of the voltage variation acrossthe terminals of the lowpass filter. This control voltage is appliedbetween the layers 12 and 30, and therefore between the contacts 62 and60. It is such that, if a variation in the control voltage of theoptical filter is tending to increase the output voltage of the lowpassfilter, the slaving filter tends to continue the variation of thecontrol voltage in the same direction, but, if a variation in thecontrol voltage of the optical filter is tending to decrease the outputvoltage of the lowpass filter, the slaving circuit tends to reverse thedirection of this variation. The control voltage of the filter (betweenthe electrodes 60 and 62) is thereby slaved to the position which givesthe maximum optical power in the detector (between the electrodes 62 and64).

The slaving circuit comprises means for establishing an approximatevoltage V_(i), representing the theoretical voltage which it isnecessary to apply to the optical filter in order to obtain tuning onchannel i in principle. The slaving circuit will deliver a voltagebetween the contacts 60 and 62, which varies around V_(i) within limitsnarrow enough not to risk latching onto a neighboring channel.

Returning to FIG. 1, the presence of spacers throughout the periphery ofthe Fabry-Pérot resonant cavity may be noted. These spacers consist ofsemiconductor layers that are not made of indium phosphide, and whichdefine the thicknesses of the air gaps between the layers of indiumphosphide.

The reason why these spacers are not made of indium phosphide is atechnological region, associated with the production of the air gaps byselective elimination of the layers deposited between two layers ofindium phosphide. A material is selected which has the same crystallattice as indium phosphide, in order to maintain epitaxial growth fromone layer to the next, but which can be eliminated selectively by anattack product which does not attack the indium phosphide. The selectedspacer material is preferably a ternary alloy InGaAs, the composition ofwhich is the one having the crystal lattice most similar to InP, namelyIn_(0.53)Ga_(0.47)As.

The fabrication of the stack of layers with spacers is carried out inthe following way: deposition of an InP epitaxial layer, then depositionof a lattice-matched InGaAs layer, followed by deposition of the secondInP epitaxial layer. Etching of this last layer according to a precisepattern (in this exemplary embodiment: disk with suspension arms) untilthe InGaAs layer is exposed. Attack of the InGaAs layer with a productthat does not attack InP, with lateral undercut etching, that is to sayInGaAs is removed even under the INP parts which are protecting it. Onlythe spacers which can be seen around the resonant cavity in the figuresare kept, and the InP layers remain suspended with air gaps interposedbetween them.

The doping of the various layers of indium phosphide and gallium indiumarsenide takes into account the facts that potentials need to be appliedthrough them, and that it is necessary to create junctions so as toapply a control potential between the contacts 60 and 62. To be able toapply a voltage between the InP layer 12 and the InP layer 30, throughthe contacts 60 and 62, the following stack structure is accordinglyprovided above the InP layer 42 which, it will be recalled, is n⁺ doped:

n⁺ doped In_(0.53)Ga_(0.47)As spacer layer

n⁺ doped InP layer 10

n⁺ doped In_(0.53)Ga_(0.47)As spacer layer

n⁺ doped InP layer 12

In_(0.53)Ga_(0.47)As spacer layer not intentionally doped; it would bebetter if this layer were completely insulating; since it is not, it isnecessary for the upper layers to be p⁺ doped in order to avoid a directshort circuit between the contacts 60 and 62. The biasing between thesecontacts is then reverse (positive on the n⁺ layers, negative on the p⁺layers);

p⁺ doped InP layer 30

p⁺ doped In_(0.53)Ga_(0.47)As spacer layer

p⁺ doped InP layer 32.

The contacts 60 and 64 on the p⁺ doped InP layers may be made of analloy based on gold and zinc; the contact 62 on an n⁺ doped InP layermay be made of gold.

To apply a tuning control voltage of the filter, a reverse-biasingpotential difference will be applied to the junctions, that is to saythe more positive potential on the contact 62 and the more negativepotential on the contact 60.

1. A tunable optical filtering component comprising, a micromachinedmonolithic structure including an optical filter; a low-absorption lightdetection element used for slaving the tuning control of the filter to awavelength received by the filter, and for transmitting the majority ofthe radiation at this wavelength.
 2. The filtering component as claimedin claim 1, wherein said filtering component includes a transparentsemiconductor substrate, on the front face of which a stack of likewisetransparent layers is formed constitutes a tunable interferometricfilter that selectively transmits the light in a narrow spectral band,centered on a wavelength which can be adjusted by an electrical voltage,the light detection element being formed on the front face of thesubstrate, between the substrate and the filter.
 3. The filteringcomponent as claimed in claim 1, wherein said filtering componentincludes a transparent semiconductor substrate, on the front face ofwhich a stack of likewise transparent layers is formed constituting atunable interferometric filter that selectively transmits the light in anarrow spectral band, centered on a wavelength which can be adjusted byan electrical voltage, the light detection element being formed abovethe filter.
 4. The filtering component as claimed in claim 1, whereinthe substrate is made of indium phosphide and the interferometric filterincludes a plurality of indium phosphide layers separated by intervalsof controlled width, at least one interval (C) of which has a width thatcan be varied under the control of an electrical voltage.
 5. Thefiltering component as claimed in claim 4, wherein the substrate is madeof not intentionally doped or semi-insulator.
 6. The filtering componentas claimed in claim 4, wherein the intervals are air gaps.
 7. Thefiltering component as claimed in claim 1, wherein the detector includesa quantum-well photodiode.
 8. The filtering component as claimed inclaim 7, characterized in that the photodiode includes a P-type dopedsemiconductor layer, a semiconductor layer that is not intentionallydoped, and an N-type doped semiconductor layer, these three layers beingsemiconductor epitaxial layers, and a very thin epitaxial layer of adifferent semiconductor alloy, which is inserted into the layer that isnot intentionally doped.
 9. The filtering component as claimed in claim8, wherein the very thin epitaxial layer is made of a material having anenergy gap of about 0.775 electron volts, corresponding to absorption inthe optical wavelength band of from 1.5 to 1.6 micrometers, thethickness of the very thin epitaxial layer being small enough to avoiddislocations of this layer in spite of the crystal lattice differencesliable to exist between this layer and the semiconductor layers whichenclose it.
 10. The filtering component as claimed in claim 9, whereinthe thickness of the very thin epitaxial layer is less than 20nanometers.
 11. The filtering component as claimed in claim 8, whereinthe photodiode is made from layers of indium phosphide, the very thinepitaxial layer being made of gallium-indium arsenide In_(x)Ga_(1-x)As,where x lies between about 0.53 and 0.63.
 12. The filtering component asclaimed in claim 1, wherein the absorption of light in the detector isof the order of from 1 to 2% at most.
 13. A method for tuning a tunableoptical component as claimed in claim 1, comprising the steps of:collecting from the detection element a signal representing a smallfraction of the light passing through this element; averaging thissignal and in producing from the average signal an electrical controlsignal for slaving the tuning of the filter; and tuning the filter at avalue which maximizes the averaged detected signal.
 14. The filteringcomponent as claimed in claim 2, wherein the substrate is made of indiumphosphide and the interferometric filter includes a plurality of indiumphosphide layers separated by intervals of controlled width, at leastone interval (C) of which has a width that can be varied under thecontrol of an electrical voltage.
 15. The filtering component as claimedin claim 5, wherein the intervals are air gaps.
 16. The filteringcomponent as claimed in claim 9, wherein the photodiode is made fromlayers of indium phosphide, the very thin epitaxial layer being made ofgallium-indium arsenide In_(x)Ga_(1-x)As, where x lies between about0.53 and 0.63.
 17. The filtering component as claimed in claim 10,wherein the photodiode is made from layers of indium phosphide, the verythin epitaxial layer being made of gallium-indium arsenideIn_(x)Ga_(1-x)As, where x lies between about 0.53 and 0.63.
 18. Thefiltering component as claimed in claim 8, wherein the absorption oflight in the detector is of the order of from 1 to 2% at most.
 19. Thefiltering component as claimed in claim 13, wherein said filteringcomponent includes a transparent semiconductor substrate, on the frontface of which a stack of likewise transparent layers is formedconstitutes a tunable interferometric filter that selectively transmitsthe light in a narrow spectral band, centered on a wavelength which canbe adjusted by an electrical voltage, the light detection element beingformed on the front face of the substrate, between the substrate and thefilter.
 20. The filtering component as claimed in claim 13, wherein saidfiltering component includes a transparent semiconductor substrate, onthe front face of which a stack of likewise transparent layers is formedconstituting a tunable interferometric filter that selectively transmitsthe light in a narrow spectral band, centered on a wavelength which canbe adjusted by an electrical voltage, the light detection element beingformed above the filter.